As a high-frequency switch circuit for passing or cutting off a high-frequency signal or for allowing a high-frequency signal to pass through, those using diodes and those using field effect transistors (FET: field effect transistor) have been conventionally known.
FIG. 1 is an example of a high-frequency switch circuit using FETs, a circuit diagram showing a high-frequency switch circuit configuration of a SPDT (single pole double through) type. Here, FIG. 1 is a circuit disposed in FIG. 1 of Japanese Patent Application Laid-open H08-139014.
The high-frequency switch circuit shown in FIG. 1 has a configuration including first switch section 121 and second switch section 122 for allowing a high-frequency signal to pass through or for cutting off a high-frequency signal.
First switch section 121 includes a plurality of FETs (four in FIG. 1) connected in series with its two ends connected to two high-frequency terminals 101 and 102. The gate terminal of each FET is connected to control terminal 111 via a resistance element. Similarly, second switch section 122 includes a plurality of FETs (four in FIG. 1) connected in series with its two ends connected to two high-frequency terminals 101 and 103. The gate terminal of each FET is connected to control terminal 112 via a resistance element. Here, high-frequency terminal 101 is shared by first switch section 121 and second switch section 122.
In the high-frequency switch circuit shown in FIG. 1, a high-level or low-level control signal is input to control terminal 111 of first switch section 121 and control terminal 112 of second switch section 122 so as to perform on/off control of first switch section 121 and second switch section 122. In this configuration, when two levels of control signals, high-level and low-level signals, are complementarily input to control terminal 111 and control terminal 112, it is possible to cause high-frequency terminal 102 or high-frequency terminal 103 to output the high-frequency signal input from high-frequency terminal 101, or cause high-frequency terminal 101 to output one of the high-frequency signals input from high-frequency terminal 102 and high-frequency 50 terminal 103.
In this high-frequency switch circuit of the prior art shown in FIG. 1, as described in paragraph 0009 in Japanese Patent Application Laid-open 2004-320439, for example, the value of the resistance between the drain and source of each FET that is in the off-state (off-state resistance) is extremely large, hence there is the problem that the potential between the drain and source terminals of each of the serially connected FETs becomes unstable.
To avoid this problem, a common practice is use of a technique that stabilizes the potential between the drain and source terminals by applying bias voltage Va to the drain terminals and source terminals of the FETs via resistance elements etc., as shown in FIGS. 2 and 3. The circuits shown in FIGS. 2 and 3 are circuits for allowing a high-frequency signal to pass through or are circuits for cutting off a high-frequency signal that passes between two high-frequency terminals T1 and T2 by turning on and off the FETs based on control signal Vc. Here, FIG. 2 is the circuit disclosed in FIG. 1 of the aforementioned Japanese Patent Application Laid-open 2004-320439 and FIG. 3 is the circuit disclosed in FIG. 1 of Japanese Patent Application Laid-open H11-239048.
In the high-frequency switch circuits using FETs as above, there is a problem in which the high-frequency signal having passed through the high-frequency switch circuit entails distortion.
Usually, the drain or source terminal of an FET provided in a high-frequency switch circuit is applied with a voltage having an amplitude in conformity with the intensity of the input high-frequency signal. At this time, in the on-state FET, the on-state resistance changes in accordance with the voltage of the input high-frequency signal while in the off-state FET, the off-state capacitance varies in accordance with the voltage of the input high-frequency signal. This phenomenon in which the FET on-state resistance and off-state capacitance vary depending on the input voltage causes distortion of the high-frequency signal that has passed through the high-frequency switch circuit.